In the packaging of certain types of foods, such as snack foods like potato chips, cookies and the like, it is common practice to employ a multi-layer film. A desirable property in such a packaging film is an opacity which protects the packaging product from deterioration caused by exposure to light. In particular, it has been found that certain wavelengths of light, up to about 450 nm cause increased spoilage in such packaged products. Even when a degree of opacity is present in the film, spoilage occurs if the film allows passage of some light.
It is known in the art that thermoplastic polymers can be loaded with inert fillers, cast into films, and thereafter stretched to form oriented thermoplastic films. While this statement is generally true, it must be realized that the particular components employed and the specific process parameters employed, particularly when control is difficult, can result in significantly different end products or contribute to the success or failure of obtaining a desired result. As an example, U.S. Pat. No. 4,118,438, discloses the use of some materials similar to those contemplated by the present invention, however, the object of U.S. Pat. No. 4,118,438 is diametrically opposed to the object of the present invention. U.S. Pat. No. 4,118,438 is concerned with a transparent polypropylene film containing fine particles of an incompatible polymer dispersed therein. The film disclosed exhibits surface projections caused by the dispersed particles and the patentees maintain that this gives the transparent film non-blocking characteristics. In U.S. Pat. Nos. 3,697,367 and 3,795,720, there is disclosed a process for preparing an uniaxially oriented mixed polymer system. The resulting material has utility as a paper substitute and can be formed into fibers for making sheet paper.
European Patent Application 87307442, filed on Aug. 24, 1987, (Publication No. 258020, published on Mar. 2, 1988), the entire contents of which are incorporated herein by reference, describes and claims a film structure which comprises: (a) a voided core layer comprising a matrix of (i) a thermoplastic polymeric material in which are dispersed (ii) void-initiating solid particles which are phase-distinct from the thermoplastic polymeric material of the matrix and about which particles are located opacifying voids and a minor amount (iii) of a light absorbing pigment; and (b) at least one transparent skin layer adhering to the surface of the voided core layer comprising a thermoplastic polymeric material which is sufficiently thick to substantially prevent the asperities of the core layer from being manifest, the light transmission of the structure being less than about 15%.
Ashcraft et al, U.S. Pat. No. 4,377,616, which is incorporated herein by reference, discloses an opaque, biaxially-oriented polymeric film structure comprising a thermoplastic core matrix having a strata of voids created by the inclusion within the matrix of spherical void-initiating solid particles incompatible with the matrix.
It is a primary object of this invention to provide films of enhanced opacity for a comparable pigment loading or of comparable opacity for a reduced pigment loading.